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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Thu, 29 Mar 2007 20:53:25 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12485345

Doug Cutting commented on HADOOP-1134:

> The client would need a way to go from a block id to a .crc file via an extension of
the Namenode API.

The split could include the datanode name, the block ID, the file name and the offset of the
block within the file.  Then the mapper could access the CRC file using normal namenode and
datanode calls.

But actually, now that I think about it, if we're primarily not validating checksums against
the data, but rather comparing all the checksums for a block, then locality may not be worthwhile.
 In that case we'd want a temporary datanode extension that permits writing the checksum file
for a block.  Then the updater map task can read through all copies of the checksum file,
construct the best possible checksum for each block, then send these to datanodes.  So, in
aggregate, 6% of the filesystem would cross the wire during the upgrade.  Could that work?

> Once a file is validated the .crc file would be deleted by the client.

Is the upgrade the time to detect corrupt blocks?  Won't these be detected through the normal
mechanisms later?  We don't want to perform any replication during the upgrade.  As a subsequent
patch, we should validate checksums during replication, so that we don't replicate a corrupt
block, but I don't think we need to do that as a part of this patch.  (Some might wait until
both patches are committed before updating a particular filesystem.)

> Block level CRCs in HDFS
> ------------------------
>                 Key: HADOOP-1134
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1134
>             Project: Hadoop
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: Raghu Angadi
>         Assigned To: Raghu Angadi
> Currently CRCs are handled at FileSystem level and are transparent to core HDFS. See
recent improvement HADOOP-928 ( that can add checksums to a given filesystem ) regd more about
it. Though this served us well there a few disadvantages :
> 1) This doubles namespace in HDFS ( or other filesystem implementations ). In many cases,
it nearly doubles the number of blocks. Taking namenode out of CRCs would nearly double namespace
performance both in terms of CPU and memory.
> 2) Since CRCs are transparent to HDFS, it can not actively detect corrupted blocks. With
block level CRCs, Datanode can periodically verify the checksums and report corruptions to
namnode such that name replicas can be created.
> We propose to have CRCs maintained for all HDFS data in much the same way as in GFS.
I will update the jira with detailed requirements and design. This will include same guarantees
provided by current implementation and will include a upgrade of current data.

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